Micro-electro-mechanical systems (MEMS) are used in many devices which require modulation of light. For example, projectors may use a digital micromirror device (DMD), which has thousands of micromirrors. A cantilever or hinged mirror 10 of this type, shown in FIG. 1, rotates on an axis 11 reflect a beam of light when in an on position. A disadvantage of this type of device is the relatively slow response time, ˜10 μs, due to the low natural frequency of each single hinged mirror.
In another type of modulation device, the deformable diffractive grating light modulation systems, the diffractive element is formed as a long narrow ribbon. In one design, the ribbon bends along the long axis of the ribbon thus forming a “piston” type switching diffractive element. Typical piston type diffractive elements are shown in U.S. Pat. Nos. 5,311,360; 5,459,610; and 5,677,783.
In another design, shown in FIG. 2a, the cross section of the diffractive ribbon is T-shaped and the ribbon bends along its short axis. FIG. 2a illustrates two pixel elements 21 and 22 of a spatial light modulator (SLM) 20 built on a silicon substrate 23, with each pixel comprised of three diffractive elements 210 and 220 respectfully. The pixels are driven using electrodes 24. Pixel 21 is shown in energized state while pixel 22 is in a non-energized state. Resulting diffraction distribution of the two pixels is shown on FIG. 2b. In energized (diffracting) state, a pixel produces symmetrical angular distribution of light intensity consisting of many diffraction maximums. T-shaped ribbon types of diffractive elements are shown, for example, in U.S. Pat. Nos. 6,661,561; 6,836,352; and 6,856,448.
Both types of diffractive elements have some advantages, while suffering from some drawbacks. In both types, however, the width of one ribbon, the pitch, determines the grating period d, and the diffracted light is distributed within multiple diffractive orders symmetrically in both directions. See FIG. 2b. The diffraction efficiency of a single element is very low with typical contrast of about 50%. Therefore, in optical systems using deformable ribbons light modulators, more than one ribbon is used for forming an optical pixel, rendering the minimum optical pixel size to two times the grating period or 2d.